The proposed study will lead to a better understanding of the pathological function of presenilin and its associated protein (PSAP) in neurodegeneration and b-amyloid formation in Alzheimer's disease (AD). The present application is driven by the following hypotheses: (1) the pathological function of presenilin 1 (PS1) is, at least in part, mediated by the functional interaction of its C-terminus with other molecule(s); (II) the interaction of PS1 with a novel protein, PSAP, identified by the applicant, plays an important role in PS1-regulated apoptosis; (III) the interaction of PSAP with PS1 influences the putative y-secretase activity of PS1, which in turn, results in the abnormal AB production; (IV) PSAP functions as a receptor of the proapoptotic protein, Bax, on the mitochondrial membrane; and (V) PSAP undergoes disease-related modification. These hypotheses are based on a number of observations: 1) the C-terrninal fragment of PS1 inhibits Fas-induced apoptosis; 2) deletion or addition of amino acids at the C-terminus of PS1 negatively affects the y-secretase activity; 3) PSAP interacts specifically with the C-terminus of PS1; 4) PSAP is localized to the mitochondrial membrane and interacts with Bax; 5) over-expression of PSAP induces apoptosis, which involves Bax translocation, cytochrome c release and caspase activation; 6) coexpression of PS1 with PSAP enhances y-secretase activity and PSAP-induced apoptosis; and 7) the C-terrninus of PSAP from AD-affected brains exhibits abnormal immunoreactivity. The goal of this application will be achieved by the following specific aims: 1) determine the effect of the interaction of PS1 with PSAP on apoptosis and y-secretase activity; 2) explore the mechanism of PSAP-induced apoptosis. 3) analyze the polymorphism and modification of PSAP and its relevance to AD. The research designs are as follows: a) the experiments designed in Aim 1 and 2 will be carried out using an inducible system expressing PSAP in neuronal N2a cells or HEK293 cells. Apoptosis will be analyzed by a series of experiments including chromosome condensation and fragmentation, DNA laddering, flow cytometry, and the cleavage of the death substrate poly(ADP-ribose) polymerase (PARP); b) the secretion and intracellular accumulation of AB (B-amyloid) and other derivatives of APP (amyloid precursor protein) will be analyzed by immunoprecipitation, Western blot, and ELISA assays in cells stably expressing APP and PS1, and transiently expressing inducible PSAP, which we have already established; and c) the tissue distribution, the polymorphism, and the possible mutations or modifications of PSAP will be analyzed by RTPCR, Western blot, and immunocytochemical analysis of samples from normal controls and AD subjects. This study may lead to the identification of a novel risk factor of AD.